The study of sleep in the marine mammals is a fascinating area of comparative sleep
physiology. Cetaceans, such as dolphins and whales (also the order Sirenia or sea cows),
must regularly come to the turbulent ocean surface to respire - this requires strict
maintenance of posture, muscle tone and reflectory activity. Because hypotonia and
hyporeflexia are impermissable conditions for Cetacean sleep, the two hemispheres
must enter the state of delta sleep alternately; always leaving one hemisphere to
maintain tone and reflex to prevent inhalation of water. In fact, paradoxical sleep
(associated with an even greater degree of hyporeflexia and hypotonia) appears to be
totally absent in Cetaceans.
The Pinnipeds (seals) are fundamentally different in that they can live and sleep in both
terrestrial and marine environments. Seals can sleep on land, floating on the water
surface or underwater, in the latter case however, they must regularly wake and come
to the surface for respiration periods. Unlike Cetaceans, they do demonstrate
paradoxical sleep and only some (such as the northern fur seal but not the Caspian
seals) exhibit unihemispheric sleep. This difference is a reflection of the fact that seals
have considerable adaptations for both terrestrial and marine environs.
The neurophysiological mechanisms for unihemispheric sleep are unclear. Dolphins
have a relatively small corpus callosum although it is thought that this factor is of little
consequence. In other mammals (such as the Opossum), the absence of a true corpus
callosum is associated with obligatory unihemispheric sleep, just as is invariably
displayed in split-brain preparations of any species.
The Cetacean brain offers unique opportunities to study aspects of sleep deprivation.
Many interesting insights into sleep physiology have been provided by such research
including the observation that absence of sleep in one hemisphere is not compensated
for by prolonged periods of sleep in the other.
Much of this work has been done by Mukhametov (with many of the reports
available only in russian) but for an interesting, albeit 'older' reference I direct you to:
Mukhametov, L.M. (1984)
Sleep in Marine Mammals.
Experimental Brain Research. Supplement 8. Berlin: Springer-Verlag. pp 227-238.
also-
Mukhametov, L.M., Supin, A.Y., and Polyakova, I.G. (1977)
Interhemispheric asymmetry of the electroencephalographic sleep patterns in dolphins.
Brain Research 134: 581-584.
I was interested to note that some correspondents to bionet.neuroscience mentioned
unihemispheric in birds. I would be most interested in any references to studies of this
kind - or indeed any recent references with relevance to the topic in general (including
work with marine mammals).
Dallas.Grasby at path.utas.edu.au